Three Tasks by syllic - Merlin/Arthur, Uther/lilac robes, Arthur/lilac robes (OKAY NOT REALLY BUT YOU CAN SORT OF SEE IT).
It's like, the most earnest and awesome thing ever that is like, the epitome of crack that goes around the corner and becomes perfectly believable. The author summarized it thus:
Arthur is the Royal Arbiter for Suitable Suitors' Disputes. He (and his trusty companion, the golden-dragon-tasselled hat) alone can determine who is fit to woo royal servants and other assorted courtiers in Uther's court.
If I had the words to describe my very near spiritual affinity for this story, I would--well, I would obviously use those, because it would be so much easier than flailing. I just--love this story. So. Much. I want canonical lilac robes and I don't like lilac. Yes, I am going there. I am embracing pastels.